1. Technical Field
The present disclosure relates to a process for hydrogenation of a middle distillate feedstock such as diesel fuel to produce improved quality diesel product.
2. Background of the Art
Petroleum distillates including gas oils boiling in the range of from about 330° F. to about 800° F., including straight run gas oils, visbreaker thermally cracked gas oil, coker gas oil, and FCC light cycle gas oil, are treated to produce improved quality diesel fuels. The diesel fuel must meet certain specifications relative to sulfur, nitrogen, olefins and aromatics content, cetane index, boiling point (distillation) and gravity. More stringent regulations will require refiners to produce ultra low sulfur content diesel (ULSD) in the coming years. Generally, this will force refiners to produce 10-50 wppm or lower sulfur content diesel fuel.
Desulfurization of hydrocarbon feedstocks by hydrotreating is known, i.e., by reacting the feedstock with hydrogen under appropriate conditions to remove the sulfur in the form of hydrogen sulfide (H2S). With recent catalyst advancements, refiners can reduce the sulfur in the treated distillate product in the existing unit, but not enough to meet the pending regulations.
Many existing hydrotreaters which are currently producing diesel fuel with sulfur levels greater than 50 wppm will require revamping and/or implementation of new units. To achieve the required diesel fuel specifications, it is necessary to treat the distillate feedstock in order to affect the chemical and physical properties of the distillates. The catalyst type and operating severity are a function of the desired diesel fuel specifications. The processing requires hydrogenation with an appropriate catalyst or a combination of different catalyst systems over a hydrogen rich environment. For sulfur, nitrogen, olefins and aromatics reduction, deep hydrogenation is required. For cetane and/or gravity improvement, both deep hydrogenation and selective ring opening is required.
Prior practice conventional processing schemes to revamp existing hydrotreaters to produce ultra low sulfur diesel will typically add a new co-current reactor in series or parallel with the existing reactor to implement additional catalyst volume. In addition, this type of revamp scheme poses significant modifications and/or replacement of existing equipment items in the high pressure reaction loop including, major piping/heat exchanger, amine scrubber, and recycle compressor. All of these existing unit modifications will result in major capital investment and down time.